1. Field of the Invention
The present invention relates to a magnetic substance which exhibits a great specific heat at extremely low temperatures.
The invention also relates to a low-temperature regenerator which exhibits excellent recuperativeness at extremely low temperatures.
2. Description of the Prior Art
In recent years, superconduction technology has remarkably advanced and has been applied to more and more technical fields. Along with the increasing use of the technology, demands are increasing for a high-efficiency, small refrigerator for cooling superconductive components. In other words, it is greatly demanded that a refrigerator be developed which is light and small and has a high heat-efficiency. At present, such refrigerators are being developed in two ways. The first method is to enhance the efficiency of the existing gas-cycle refrigerator by adopting, for example, the Stirling cycle. The second method is to employ new refrigeration system in place of the conventional gas-cycle refrigeration. The new refrigeration system includes heat-cycle using magnetocaloric effect, such as a Carnot-type and an Ericsson-type cycle.
Among the gas-cycle refrigerators with enhanced efficiency are: a refrigerator which operates in the Strirling cycle; a refrigerator which operates in the Vuilleumier cycle; and a refrigerator which operates in the Gifford-Mc Mahon cycle. Each of these refrigerators has a regenerator packed with regenerative materials. A working medium is repeatedly passed through the regenerator, thereby obtaining a low temperature. More specifically, the working medium is first compressed and then made to flow in one direction through the regenerator. As the medium flows through the regenerator, heat energy is transferred from the medium to the generative materials. Thus, the working medium is deprived of heat energy. When the medium flows out of the regenerator, it is expanded to have its temperature lowered further. The working medium is then made to flow in the opposite direction, through the regenerator again. This time, heat energy is transferred from the regenerative materials to the medium. The medium is passed twice, back and forth, through the regenerator in one refrigeration cycle. This cycle is repeated, thereby obtaining a low temperature.
The recuperativeness of the generative materials is the determinant of the efficiency of the refrigerator. The greater the recuperativeness the generative materials have, the higher the heat-efficiency of each refrigeration cycle.
The regenerative materials used in the conventional regenerators are particles of lead or bronze particles, or nets of cupper or phosphor bronze. These regenerative materials exhibit but a very small specific heat at extremely low temperatures of 20 K or less. Hence, they cannot sufficiently accumulate heat energy at extremely low temperatures, in each refrigeration cycle of the gas-cycle refrigerator. Nor can they supply sufficient heat energy to the working medium. Consequently, any gas-cycle refrigerator which has a regenerator filled with such regenerative materials fails to obtain an extremely low temperatures.
This problem can be solved by using regenerative materials which exhibit a great specific heat per unit volume (i.e., volume specific heat) at extremely low temperatures. Much attention is paid to some kinds of magnetic substances as such regoenerative materials, since they exhibit magnetocaloric effect, that is, their specific heats greatly change at their magnetic transition temperatures. Hence, any mnagnetic substance, whose magnetic transition temperature is extremely low, can make excellent regenerative materials.
One of such magnetic substances is the R-Rh intermetallic compound (where R is Sm, Gd, Tb, Dy, Ho, Er, Tm, or Yb) disclosed in Japanese Patent Disclosure No. 51-52378. This compound has a maximal value of volume specific heat which is sufficiently great at 20 K or less.
One of the components of this intermetallic compound is rhodium (Rh). Rhodium is a very expensive material. In view of this, it is not suitable as a component of regenerative materials which are used in a regenerator, in an amount of hundreds of grams.
The R-Rh intermetallic compound has a small volume specific heat at temperatures higher than 20 K. This is because the compound has but a small lattice specific heat. The lattice specific heat is largely responsible for the volume specific heat of the compound unless the volume specific heat increases due to the magnetocaloric effect. Hence, other regenerative materials must be used to obtain a low temperature down to 20 K in a gas-cycle refrigerator system utilizing the R-Rh intermetallic compound.
Conventionally, copper is used as the regenerative material for cooling from room temperature down to about 40 K, and lead is used as the regenerative material for cooling from 40 K down to about 20 K. Therefore, in order to obtain an extremely low temperatures of less than 20 K in a refrigerator system utilizing the R-Rh intermetallic compound, the three different regenerative materials (Cu, Pb and R-Rh compound) will have to be successively used in accordance with the temperature ranges to which the refrigerator system reaches.